Do animals experience emotions? Of course they do, and solid science, combined with countless stories, show this to be so. It is bad science to rob animals of their feelings. When someone tells me they are not sure if dogs, for example, experience joy or grief, I say I'm glad I'm not your dog.

This week's picture of Gana, the 11-year-old gorilla shown grieving the loss of her infant, was one of the most poignant images I have ever seen of an animal in distress.

Soon after it appeared, my email inbox exploded with messages and questions about Gana and the nature of animal emotions.

As a scientist who has studied animal emotions for more than 30 years, I consider myself extremely fortunate. I love learning about animals and sharing what my colleagues and I discover. The photograph of Gana reminded me of Flint, a young male chimpanzee who died soon after his mother, Flo. In a heartbreaking series of events, the primatologist Dame Jane Goodall observed Flint withdraw from his group, stop feeding, and finally die.

"Flint became increasingly lethargic, refused food and, with his immune system thus weakened, fell sick," she wrote in her book, Through a Window. "The last time I saw him alive, he was hollow-eyed, gaunt and utterly depressed, huddled in the vegetation close to where Flo had died. The last short journey he made, pausing to rest every few feet, was to the very place where Flo's body had lain. There he stayed for several hours, sometimes staring into the water. He struggled on a little further, then curled up – and never moved again."

The Nobel-winning zoologist Konrad Lorenz also observed grief in geese that was similar to grief in young children. "A greylag goose that has lost its partner shows all the symptoms described in young human children," he wrote. "The eyes sink deep into their sockets, and the individual has an overall drooping experience, literally letting the head hang." Grief is the price of deep commitment, and surely no one would dispute that deep commitments to loved ones are necessary in all animal groups.

Since we already know so much about the emotional lives of diverse species, arguments to the contrary are often excuses to retain the status quo – in other words, human superiority.

The real question is why emotions have evolved, not if they have evolved.

In a nutshell, emotions have evolved with adaptations in numerous species. These adaptations serve as a social glue to bond animals with one another. Emotions also catalyse and regulate a wide variety of social encounters among friends and competitors and permit animals to behave adaptively and flexibly in a wide variety of venues. Animal emotions allow individuals to move through the world with likes and dislikes, just as we do.

Scientists often discount the opinions of those who claim animals feel emotions as overly anthropomorphic, dismissing their supporting anecdotes as trivial. None the less, many researchers recognise that we must be anthropomorphic when discussing animal emotions.

That animals and humans share many traits including emotions is merely an extension of Charles Darwin's accepted ideas about evolutionary continuity, that the differences between species are differences in degree rather than differences in kind. The seemingly natural human urge to impart emotions on to animals, far from obscuring the "true" nature of animals, may actually reflect a very accurate way of knowing.

What we know about animal emotions goes beyond mere dinner-party anecdote. But stories do count. The plural of anecdote is data. Thus, after I published my own observations of a magpie funeral ceremony, in which individuals paid tribute to their dead friend by standing silently around her, touching the corpse lightly, before flying off and bringing back grass to place down by the body, I had a slew of emails from people who had seen the same ritual in crows and ravens.

The same thing happened when I watched a female fox bury her mate after a cougar near my home had killed him. And while these stories are indeed data, it is good to see that they and others are being constantly validated by more controlled scientific endeavours.

What excites me about science is that there are always surprises. Just when we think we've seen it all, new scientific data appears that forces us to rethink what we know and revise our stereotypes.

Anyone who has worked with whales knows they are emotional giants. Spindle cells, long thought to exist only in humans and other great apes, have recently been discovered in humpback, fin, killer and sperm whales in the same area of their brains as spindle cells occur in human brains.

This brain region is linked with social organisation, empathy and intuition about the feelings of others, as well as rapid gut reactions. Spindle cells are important in processing emotions. It is likely that if we seek the presence of spindle cells in other animals, we will find them.

Along these lines, who would have thought that laboratory mice are empathic rodents? But now we know they are. Research has shown that mice react more strongly to painful stimuli after they have observed other mice in pain, and it turns out that they're fun-loving as well.

Neurobiologist Jaak Panksepp has shown that rats like to play and be tickled. Recent research shows that birds and fish are also sentient and experience pain and suffering. Many animals know right from wrong and display moral behaviour – what I call "wild justice".

Neuroscientific research has shown that all mammals, including humans, share neuroanatomical structures and neurochemical pathways in the limbic system that are important for experiencing emotions. It has also shown that elephants have a huge hippocampus, a brain structure that is important in processing emotions.

Consistent with these findings, we know that elephants are highly emotional beings that suffer from psychological flashbacks and they are thought to experience the equivalent of post-traumatic stress disorder. They even grieve the loss of friends.

In many ways, science is catching up with what we already knew about animal passions. The number of sceptics is dwindling because of the flurry of research on the emotional lives of animals published in the world's best science journals.

I often begin my lectures by asking, "Is there anyone here who thinks that dogs don't have feelings, that they don't experience joy and sadness?"

I've never had an enthusiastic response to this question, even in scientific gatherings, although on occasion a hand or two goes up slowly, usually halfway, as the person glances around to see if anyone is watching. But if I ask, "How many of you believe that dogs have feelings?" almost every hand waves wildly and people smile and nod in agreement.

Our relationship with other animals is a complex, ambiguous, challenging and frustrating affair, and we must continually reassess how we interact with our non-human kin. After all, they have feelings too, as the picture of Gana this week proved beyond question.

----- Marc Bekoff is Professor Emeritus at the University of Colorado and has written several books including The Emotional Lives of Animals